1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
|
/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** Contact: Qt Software Information (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:LGPL$
** No Commercial Usage
** This file contains pre-release code and may not be distributed.
** You may use this file in accordance with the terms and conditions
** contained in the either Technology Preview License Agreement or the
** Beta Release License Agreement.
**
** GNU Lesser General Public License Usage
** Alternatively, this file may be used under the terms of the GNU Lesser
** General Public License version 2.1 as published by the Free Software
** Foundation and appearing in the file LICENSE.LGPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU Lesser General Public License version 2.1 requirements
** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
**
** In addition, as a special exception, Nokia gives you certain
** additional rights. These rights are described in the Nokia Qt LGPL
** Exception version 1.0, included in the file LGPL_EXCEPTION.txt in this
** package.
**
** GNU General Public License Usage
** Alternatively, this file may be used under the terms of the GNU
** General Public License version 3.0 as published by the Free Software
** Foundation and appearing in the file LICENSE.GPL included in the
** packaging of this file. Please review the following information to
** ensure the GNU General Public License version 3.0 requirements will be
** met: http://www.gnu.org/copyleft/gpl.html.
**
** If you are unsure which license is appropriate for your use, please
** contact the sales department at qt-sales@nokia.com.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\page qtbinding.html
\target qtbinding
\title Using QML in C++ Applications
\tableofcontents
The QML API is split into three main classes - QmlEngine, QmlComponent and QmlContext.
QmlEngine provides the environment in which QML is run, QmlComponent encapsulates
\l {QML Documents}, and QmlContext allows applications to expose data to QML component instances.
QML also includes a convenience API, QmlView, for applications that simply want to embed QML
components into a new QGraphicsView. QmlView covers up many of the details discussed below.
While QmlView is mainly intended for rapid prototyping it can have uses in production applications.
\section1 Basic Usage
Every application requires at least one QmlEngine. A QmlEngine allows the configuration of
global settings that apply to all the QML component instances - such as the QNetworkAccessManager
that is used for network communications, and the path used for persistent storage.
Multiple QmlEngine's are only needed if the application requires these settings to differ
between QML component instances.
\l {QML Documents} are loaded using the QmlComponent class. Each QmlComponent instance
represents a single QML document. A QmlComponent can be passed a document URL, or raw text
representing the content of the document. The document URL can be a local filesystem URL, or
any network URL supported by QNetworkAccessManager.
QML component instances can then be created by calling the QmlComponent::create() method. Here's
an example of loading a QML document, and creating an object from it.
\code
QmlEngine *engine = new QmlEngine(parent);
QmlComponent component(engine, QUrl("main.qml"));
QObject *myObject = component.create();
\endcode
\section1 Exposing Data
QML components are instantiated in a QmlContext. A context allows the application to expose data
to the QML component instance. A single QmlContext can be used to instantiate all the objects
used by an application, or several QmlContext can be created for more fine grained control over
the data exposed to each instance. If a context is not passed to the QmlComponent::create()
method, the QmlEngine's \l {QmlEngine::rootContext()}{root context} is used. Data exposed through
the root context is available to all object instances.
\section1 Simple Data
To expose data to a QML component instance, applications set \l {QmlContext::setContextProperty()}
{context properties} which are then accessible by name from QML \l {Property Binding}s and
\l {ECMAScript Blocks}. The following example shows how to expose a background color to a QML
file.
\table
\row
\o
\code
// main.cpp
QmlContext *windowContext = new QmlContext(engine->rootContext());
windowContext->setContextProperty("backgroundColor",
QColor(Qt::lightsteelblue));
QmlComponent component(&engine, "main.qml");
QObject *window = component.create(windowContext);
\endcode
\o
\code
// main.qml
import Qt 4.6
Rectangle {
color: backgroundColor
Text {
anchors.centerIn: parent
text: "Hello Light Steel Blue World!"
}
}
\endcode
\endtable
Context properties work just like normal properties in QML bindings - if the \c backgroundColor
context property in the previous example was changed to red, the component object instances would
all be automatically updated.
QmlContexts form a tree - each QmlContext except for the root context has a parent. Child
QmlContexts effectively inherit the context properties present in their parents. This gives
applications more freedom in partitioning the data exposed to different QML object instances.
If a QmlContext sets a context property that is also set in one of its parents, the new context
property shadows that in the parent. In The following example, the \c background context property
in \c {Context 1} shadows the \c background context property in the root context.
\image qml-context-tree.png
\section2 Structured Data
Context properties can also be used to expose structured and writable data to QML objects. In
addition to all the types already supported by QVariant, QObject derived types can be assigned to
context properties. QObject context properties allow the data exposed to be more structured, and
allow QML to set values.
The following example creates a \c CustomPalette object, and sets it as the \c palette context
property.
\code
class CustomPalette : public QObject
{
Q_OBJECT
Q_PROPERTY(QColor background READ background WRITE setBackground NOTIFY backgroundChanged)
Q_PROPERTY(QColor text READ text WRITE setText NOTIFY text)
public:
CustomPalette() : m_background(Qt::white), m_text(Qt::black) {}
QColor background() const { return m_background; }
void setBackground(const QColor &c) {
if (c != m_background) {
m_background = c;
emit backgroundChanged();
}
}
QColor text() const { return m_text; }
void setText(const QColor &c) {
if (c != m_text) {
m_text = c;
emit textChanged();
}
}
private:
QColor m_background;
QColor m_text;
};
int main(int argc, char **argv)
{
// ...
QmlContext *windowContext = new QmlContext(engine->rootContext());
windowContext->setContextProperty("palette", new CustomPalette);
QmlComponent component(&engine, "main.qml");
QObject *window = component.create(windowContext);
}
\endcode
The QML that follows references the palette object, and its properties, to set the appropriate
background and text colors. When the window is clicked, the palette's text color is changed, and
the window text will update accordingly.
\code
// main.qml
import Qt 4.6
Rectangle {
width: 240
height: 320
color: palette.background
Text {
anchors.centerIn: parent
color: palette.text
text: "Hello Colorful World!"
}
MouseRegion {
anchors.fill: parent
onClicked: {
palette.text = "blue";
}
}
}
\endcode
To detect when a C++ property value - in this case the \c CustomPalette's \c text property -
changes, the property must have a corresponding NOTIFY signal. The NOTIFY signal specifies a signal
that is emitted whenever the property changes value. Implementers should take care to only emit the
signal if the value \e changes to prevent loops from occuring. Accessing a property from a
binding that does not have a NOTIFY signal will cause QML to issue a warning at runtime.
\section2 Dynamic Structured Data
If an application is too dynamic to structure data as compile-time QObject types, dynamically
structured data can be constructed at runtime using the QmlPropertyMap class.
\section1 Network Components
If the URL passed to QmlComponent is a network resource, or if the QML document references a
network resource, the QmlComponent has to fetch the network data before it is able to create
objects. In this case, the QmlComponent will have a \l {QmlComponent::Loading}{Loading}
\l {QmlComponent::status()}{status}. An application will have to wait until the component
is \l {QmlComponent::Ready}{Ready} before calling \l {QmlComponent::create()}.
The following example shows how to load a QML file from a network resource. After creating
the QmlComponent, it tests whether the component is loading. If it is, it connects to the
QmlComponent::statusChanged() signal and otherwise calls the \c {continueLoading()} method
directly. This test is necessary, even for URLs that are known to be remote, just in case
the component has been cached and is ready immediately.
\code
MyApplication::MyApplication()
{
// ...
component = new QmlComponent(engine, QUrl("http://www.example.com/main.qml"));
if (component->isLoading())
QObject::connect(component, SIGNAL(statusChanged(QmlComponent::Status)),
this, SLOT(continueLoading()));
else
continueLoading();
}
void MyApplication::continueLoading()
{
if (component->isError()) {
qWarning() << component->errors();
} else {
QObject *myObject = component->create();
}
}
\endcode
*/
|
